The present invention relates to a heavy duty pneumatic radial tire and, more particularly, a heavy duty pneumatic radial tire improved in or relating to the impact resistance and the durability without the weight of the tire increased and without a sacrifice caused to the steering stability.
Generally, in heavy duty pneumatic radial tires (hereinafter referred to simply as heavy duty tires), their tread portion is reinfoced with at least two belt layers comprising steel cords.
Such heavy duty tires of which the tread portion is reinforced with belt layers comprising steel cords as above have a remarkable steering stability and, in addition, have remarkable tire performance characteristics such as a high abrasion resistance and a low fuel consumption characteristic. However, steel cords have an essential shortcoming that when subjected to contact with water, they undergo rusting and corrosion, and a problem is indicated that when water is permitted to enter for example a cut damage in the tread portion and reach the belt layer, it tends to occur that the water permeates into the interior of the steel cords in the belt layer and cause them to corrode, resulting in that the adhesion between the steel cords and a coat rubber becomes deteriorated to generate a separation of the tread and the belt layer from each other.
To give solution to the above problem, the U.S. Pat. No. 4,715,419 (Kawasaki et al) proposes to provide a heavy duty tire by using an element wire shaped prior to a cord formation to provide a highly extensible steel cord preferably of 2+7 or 2+6 construction and having an increment of an initial elongation (P.L.E.) of 0.3 to 1.5% when a load is increased from 0.25 kg to 5 kg and an elongation at break of 3.0 to 5.5%, and then by utilizing the above obtained steel cord in an outermost belt layer. The above steel cord permits an easy permeation of a coat rubber into its interior, so that it can exhibit a remarkable corrosion resistance. However, the P.L.E. of the cord is so large that the strength bearing ratio of the outermost belt layer is extremely limited, so that this belt layer cannot exhibit a satisfactory reinforcing function, and also the rigidity of the outermost belt layer incorporating this steel cord tends to be lowered, whereby the steering stability of the tire is lowered.
Then, as proposed in the above referred-to U.S. Pat. No. 4,715.419 for example, conventionally it has been generally practiced to use a relatively thick or bold steel cord in an inner belt layer to thereby make up for the insufficiency in the strength or the rigidity. However, this measure is not advantageous in that the weight of the tire is then increased, indispensably. That is to say, conventionally made attempts to improve the impact resistance and the durability of tires are generally such as to increase the P.L.E. of the steel cords for use in or for the outermost belt layer and thereby let escape the strain that the tire will receive. However, by increasing the P.L.E. of the steel cords for the outermost belt layer as above, the strength bearing ratio of the outmost belt layer becomes lowered, and because of this, it becomes indispensable to increase the diameter of the element wire for the steel cord for an inner belt layer in order to compensate for the lowering of the strength bearing ratio of the outermost belt layer so as to increase the strength at break. As a result of this, an increase in the weight of the tire is unavoidable.